Beachrock formation in low latitudes: implications for coastal evolutionary models

Beachrock is present in many parts of the world, particularly on low-latitude coastlines. Its formation during stillstands or minor regressions in subtropical areas involves cementation of clastic grains and thus prevents grain transport in the littoral zone during later transgression or regression. Existing short and medium-term conceptual and numerical models of coastal evolution fail to take into account this effective reduction of littoral sand supply and the effects of the beachrock itself on shoreline development. In the short term, reduction of sediment supply through beachrock formation during transgression should enhance landward translation of the shoreline: conversely the beachrock itself may afford a degree of protection from erosion. In the medium to long term, beach-rock formation during multiple Pleistocene transgressions and regressions in areas which lack significant sediment input from land drainage systems may have caused a long-term reduction in sediment volume during the course of the Pleistocene. (This assumes that beachrock formation is not matched by its destruction through physical and bioerosion and chemical weathering.) Preservation of coastal facies is greatly enhanced by beachrock formation which may promote overstepping and drowning of strandline facies during transgression or preferential preservation during regression. Precise records of sea-level fluctuations may be preserved by beachrock formation, a situation rendered almost impossible in other areas due to littoral transport of mobile sediment. In the widespread areas in low latitudes where it occurs, beachrock formation may be an important element in models of horizontal shoreline displacement and coastal evolution.